Method of bonding layers to an austenitic chromium steel core



March 12, 1968 G. H. FRIELINGQ JR METHOD OF BONDING LAYERS TO AN AUSTENITIC CHROMIUM STEEL CORE Filed May 5, 1965 United States Patent 3,372,465 METHOD OF BGNDING LAYERS TO AN AUSTENITIC CHRGMIUM STEEL CORE Gerald H. Frieling, Jr., Barrington, 12.1., assignor to Texas Instruments Incorporated, Dallas, Tex., a

corporation of Delaware Filed May 3, 1965, Ser. No. 452,767 3 Claims. (Cl. 29--474.1)

ABSTRACT OF THE DISCLOSURE An austenitic chromium steel core, from the surface of which chromium oxide has been removed, in a cleaning solution back-plated in the solution with a solidphase bondable chromium-free metal, such as copper or a precious metal such as, for example, silver, gold or the like. Clean strips of a cupronickel alloy or titanium are positioned on opposite sides of the plated core and moved through a reducing atmosphere, and thence directly into the bite between squeezing rolls. The core and strips are rapidly heated in their passage through the reducing atmosphere. The rolls squeeze the strips around the plated core to effect solid-phase bonding between the strips and the plating. The margins of the strips are also squeezed together and become solid-phase bonded to one another. Upon emergence of the clad material from the rolls, it is at once quickly cooled in a nitrogen atmos phere. Waste beyond the pinch lines is removed. By means of quick heating and quick cooling, excessive precipitation in the steel of detrimental carbides is avoided and the strength of the steel preserved. The plating shields the steel core against the inherently instantaneous oxidation of the steel surface to chromium oxide when exposed to air and substitutes the more slowly oxidizing surface of the copper or precious metal for effective solid-phase bonding under pressure. The resulting cupronickel or titanium surface of the resulting clad strand is corrosion resistant and in the case of the cupronickel is antifouling because of the cupric ions provided thereby when used for making submarine cables or the like.

This invention relates to corrosion-resistant or anticorrosion clad metal wire or rod-like forms and the like, hereinafter referred to as strands, and with regard to certain more specific features, to such strands useful in corrosive mediums. The term strands shall be understood herein to include wires, rods and the like for use by way of example in constructing cables for marine hoisting and submarine dragging uses. Qther uses are not precluded, such as in land-based hoisting, dragging equipment, bridge cables, et cetera.

Among the several objects of the invention may be noted the provision of a tough strand having a stainless steel or like core and a cupronickel or like cladding metallurgically bonded thereto in the solid phase; the provision of a method for constructing such a strand which minimizes the degradation of the surfaces to be metallurgically bonded before they become bonded, whereby high bonding strength is attained; and the provision of a method of the class described which may be economically carried out to produce a superior substantially noncorrodible strong product having minimal embrittling components in its structure. Other objects and features will be in part apparent and in part pointed out hereinafter.

The invention accordingly comprises the elements and combinations of elements, substances and combinations of substances, the proportions thereof, steps and sequence of steps, features of construction, composition and manipulation, and arrangements of parts which will be exemplifled in the constructions, products and methods hereinafter described, and the scope of which will be indicated in the following claims.

In the accompanying drawings, in which several of various possible embodiments of the invention are illustrated,

FIG. 1 is a schematic view illustrating the process;

FIG. 2 is an enlarged cross section taken on line 2-2 of FIG. 1, certain copper plating being shown by dots;

FIG. 3 is an enlarged cross section taken on line 33 of FIG. 1; and

FIG. 4 is an enlarged cross section taken on line 44 of FIG. 1, illustrating the finished product.

Corresponding reference characters indicate corresponding parts throughout the several views of the drawings.

Multi-strand cables are composed of strands of various proportions formed of steel suitably extended twisted and/or woven to produce the tensile strength required of the cable. The strands of such cables are often subjected to corrosion, both in the atmosphere and, particularly in the case of marine applications, by sea water and organisms therein.

It has been proposed to employ for each of such strands a strong stainless steel or like austentic core having a cladding of cupronickel or the like, metallurgically bonded thereto preferably by a solid-phase bonding process such as shown in general in United States Patent 2,753,623. Austenitic stainless steels and similar alloys, as for example SAE 301, 301L, 304, 304L, 305, 316, 31-6L and AM 355, have chromium contents ranging from 12 to 18%. These will precipitate carbides when either heated or cooled slowly through an 800 F. to 1500" F. temperature range. Carbides are detrimental to the end product because they cause brittleness and when unprotected accelerate corrosion. By means of the present invention this eventuality is minimized or avoided. While all of the above-designated chromium-containing steels or the like are strong and useful for purposes of the invention, low carbon stainless steels are preferred because they will precipitate carbides less readily than the others. Also stainless steel provides satisfactory tensile strength. Certain stainless steels are nonmagnetic, a useful property in some cases.

Another feature of the invention is the avoidance of degradation by surface oxidation to chromium oxide caused by any chromium content of the stainless steel before the solid-phase bonding step is reached, thereby to gain better conditions for bonding. The stainless steels when exposed to air instantaneously form chromium oxide coatings, which poses a problem in manufacture, to be referred to below.

Referring now more particularly to the drawings, there is shown at numeral 1 a copper-clad stainless steel strand of wireor rod-like form which may, for example but without limitation, have a diameter of .312 inch. This is prepared by annealing the steel strand and then cleaning it by immersion in acid, the acid forming an electroplating bath which, before the clean strand is removed, is back-plated in the acid with a thin copper layer 5. By back-plating is meant that the strand, which is cleaned by the acid, is electroplated with copper in the same acid before removal therefrom, thereby preventing access of air to the steel before plating, which would bring about rapid surface oxidation to chromium oxide. The thickness of the plating is not critical, except that it should be built up sufiiciently to substantially minimize holes at which chronium oxide might form. Thus any chromium oxide on the surface of the strand when it goes into the bath is removed and replaced by a copper surface layer which thereafter protects against chromium oxide formation. Any copper oxidation that may then occur on the core will be in the form of copper oxide, which is more on; slowly formed and more readily removed than chromium oxide. Moreover, the copper surface on the stainless steel core constitutes an ideal metal when clean for the subsequent solid phase bonding steps, including diffusion into certain cupronickel cladding to be employed.

At numerals 3 are shown two degreased, cleaned strips or ribbons of cupronickel alloy having, for example, a copper-nickel content ranging from 70 to 30% to 90 to 10%. These have been suitably worked by cold rolling or the like to approximately a semihard state (quarter hard, for example). Each strip (FIG. 2) is on the order of .012 inch thick and .625 inch wide. Cupronickel alloys are relatively low on the galvanic series of metals and alloys. Hence they are relatively inactive in sea water or other corrosive atmosphere. In addition, they possess strong antifoulant capability in that they give oif cupric ions which are poisonous to living organisms, and prevent the formation of such organisms on the surface of the strands. Thus the cables made according to the invention are more easily handled and the tendency to form crevices is reduced. While the abovementioned cupronickel alloys are preferred, titanium may be employed.

The strips 3 slide through electrical contacts 7 and the copper-coated core 1 slides through an electrical contact 9. They then proceed through a retort 11 in which is circulated a reducing atmosphere of disassociated ammonia. Heretofore, in the absence of a copper coating such as on the stainless steel, such a reducing atmosphere was required to be held at an excessively low dew point (for example, minus 60 F.) to minimize the formation of water which would interfere with the chromium oxide reducing reaction. With the copper coating, a higher dew point may be used (for example, plus 50 to 60 F.) to minimize the formation of water which would interfere with the copper oxide reducing reaction. Such a higher dew point is easier to maintain.

From the retort 11, the core 1 and strips 3 converge to a bite between pressure squeezing rolls 13 of a rolling mill 15. The cross sections of the rolls at their bite are illustrated in FIG. 3. At numeral 17 is shown an electrical control system having leads 19 passing to the contacts 7 and 9, respectively. A circuit-closing lead 21 is grounded on one of the rolls 13. By suitable known control circuitry in the box 17, currents are applied to each of the strips 3 on the one hand, and through the core 1 on the other hand, these being adjusted preferably to maintain a temperature of approximately 1700 F. in the cupronickel strips 3, and a temperature of approximately 1800 F. in the core 1. An acceptable temperature range for the cupronickel strips 3 is 1500 F. to 1800 F. to avoid melting them. An acceptable range for the core 1 is 1550 F. to 1850 F.

In view of the above, it will be seen that the disassociated ammonia in retort 11 efiiciently reduces any copper oxides that may have slowly formed on the cupronickel strips before entering the retort 11. The distance between the bite between the rolls 13 and the outlets from the retort 11 is small, so that the amount of oxidation that slowly occurs in the interval of advance to the rolls before squeezing is also small and is quickly removed. Squeezing is accomplished with a reduction in the range of to with 12% preferable. The result is that strips 3 are formed around the core 1 and solid-phase bonded thereto under pressure and nonmelting heat. The strips are likewise solid-phase bonded to one another along strips indicated at 23. The rolls 13 have rolling contact at lands 25 and a small clearance of .010 inch or less where the bonding strips 23 are to occur. The result is a pinch-off of waste material 27 which may fall away, as indicated. In cases where the clearance does not result in a complete pinch-off, the waste may be removed by any suitable means known to those skilled in the art in order to obtain a strand without side flashing. The finished form of the cladding is shown at 29 in FIG. 4.

As known to those skilled in the art, solid-phase bonding by means of pressure rolls requires not only substantial reductions during squeezing, which accounts for socalled green bonding, but also a heat treatment (at a temperature below the liquid phase) to improve the bond. Such heat treatment may come after squeezing or during squeezing. In the present case the temperature occurs during squeezing by reason of the strips 3 and the core 1 having passed through the heated retort 11. The time of heating is short, as for example 40 seconds, or a minute at most. The significance of this will appear. The temperature is substantially maintained up to the roll bite.

The clad material leaving the rolls 13 enters a waterjacketed cooling retort 31 containing a nitrogen atmosphere through which the material passes within about 50 seconds or a minute at most. The exit temperature of the material moving from the retort 31 is on the order of 250 F. At numeral 33 a water spray is applied to effect final rapid cooling to about 100 F after which the material is rolled up in a coil 35, where it assumes room temperature. If any undesirable side flashing remains on the finished product, it may be removed by means known in the art, as by skiving, for example but without limitation.

The significance of the short heating time in retort 11 (40 seconds) and the short cooling in 50 seconds in the cooling retort 31 is to minimize excessive precipitation of carbon from the austenitic steel. Austenitic stainless steel will precipitate carbides if it is either heated or cooled slowly through an 800 F. to 1500 F. temperature range. Thus, by passing rapidly through this range both during the heating, pressure bonding and cooling processes, carbide precipitation is minimized and the formation of the Sigma phase during cooling is prevented. It will also be recalled that the preferred stainless steels having a lower carbon content are also useful. carbides are detrimental to the end product in that they cause brittleness and accelerate corrosion that may set in at any defect.

The stainless steel of the core 1 is in a fully annealed condition. Thus its structure is in solid solution. A temperature of from 1850 F. to 1900 F. would take it out of solid solution. Thus the temperatures in retort 11 do not reach such values.

It will be understood that, although the layer 5 is disclosed as copper, it might be any other metal suitable for forming a solid-phase bond but one which is chromiumfree, such as silver, gold or, in fact, any of the precious metals.

It will also be understood that the drawings are illustrative and not to scale.

In view of the above, it will be seen that the several objects of the invention are achieved and other advantageous results attained.

As various changes could be made in the above constructions, product and methods without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. The method of making a corrosion-resistant clad metal strand, comprising cleaning and copper-plating a core-forming length of annealed austenitic stainless steel, said cleaning and plating being accomplished by immersion in an acid plating bath and back-plating with copper in said bath, the copper plating protecting the surface against oxidation of chromium, locating said length of stainless steel between lengths of clean strips of a cupronickel alloy having a copper content in the range of approximately 70 to and a nickel content in the range of approximately 10 to 30%, moving the core and strips through heating means to the bite between squeezing rolls for pressing together and substantially transversely reducing said lengths while heated with a reduction in the range of from 10 to 15% to produce a solid-phase bond between the core and the strips and between the strips themselves to form an integrated clad metal strand, said heating of the core material being at a temperature in the range of from approximately 1500 F. to approximately 1850 F. and of the strips in the range of from approximately 1500 F. to approximately 1800" F., and advancing the integrally bonded core and strips from the reducing means to and through a cooler, the heating and cooling times being on the order of a minute or less to minimize precipitation of carbon from the solid solution of the steel.

2. The method as set forth in claim 1 wherein said stainless steel is selected from the group consisting of SAE 301, 301L, 304, 304L, 305, 316, 316L and AM 355.

3. The method of making a corrosion-resistant clad metal strand, comprising cleaning chromium oxide from a core-forming wire of austenitic chromium steel and before reformation of such oxide plating the strand with a metal which oxidizes more slowly than does said chromium steel, the plated metal being selected from 'the group consisting of copper and the precious metals, placing the length of plated steel between lengths of clean metal strips selected from the group consisting of cupronickel alloy and titanium and passing all of said lengths rapidly through a heated reducing atmosphere and through squeezing means to squeeze together said lengths while heated to effect a solid-phase bond between the plating on the core and the strips and also between the margins of 'the strips themselves to form an integrated solid-phase bonded clad metal strand, and rapidly advancing the integrally bonded core and strips from the squeezing means to and through cooling means, the heating and cooling periods being on the order of a minute or less to minimize precipitation of carbides in the steel.

References Cited UNITED STATES PATENTS 2,746,135 5/1956 Harris 29-196.3 X 2,753,623 7/1956 Boessenkool 29-497.5 2,834,102 5/ 1958 Ptlumm 29-494 X 2,908,073 10/1959 Dulin 29-497.5 X 3,220,106 11/1965 Clark 29-497.5 3,247,082 4/1966 Du Rose 29-196.3 X 3,251,660 5/ 1966 Finsternwalder 29-196.3

HYLAND BIZOT, Primary Examiner. 

